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Octocoral colonies at East Cabritte and Europa Bay were surveyed to assess the spatial distribution and,in particular, the incidence of colonies in close proximity to each other. At each site, five 10 m × 1 m parallel belt transects were set up perpendicular to shore at10 m intervals starting at an arbitrarily selected point. Erythropodium caribaeorum and the encrusting form of Briareum asbestinum were not included in the surveys because they are not branching and, therefore,do not compete for space in the canopy. Each octocoral ≥5 cm in height was identified to species level in the field when possible; otherwise a small, 3 cm long sample was collected from colonies ≥15 cm tall for sclerite examination under a microscope. Images of the colony and a close-up image of the collected branch were also obtained. Identifications were based on Bayer (1961) and Sánchez (2009).

Colonies were divided into 2 classes, those in close proximity to a neighbor, cases in which a colony’s branches or base were within 5 cm of another octocoral colony, and those more distantly spaced. When the branches of 2 colonies are <5 cm apart, branch movement driven by currents and wave action often leads to contacts between the branches. Large colonies whose bases are within 5 cm of each other almost always have branches within 5 cm of each other, and small colonies with bases <5 cm apart at the time of settlement will almost inevitably come into contact with each other as they grow in the canopy. We recorded all instances of interactions. We did not distinguish between interspecific and intraspecific interactions since our goal was to first determine the overall incidence of competition among branching octocorals at the 2 sites.

At East Cabritte, which had a greater density of octocorals than Europa Bay, 4 randomly selected 1×1 m2 quadrats on each transect were surveyed while all 10 quadrats on each transect were surveyed atEuropa Bay. The height of each colony was measured to the nearest centimeter. Each colony was assessed for proximity to adjacent colonies and for effects of proximity, tissue damage to branches in close proximity to an adjacent colony and/or an asymmetric colony form. Only the octocorals with an asymmetric colony form attributable to the presence of another octocoral were scored as asymmetric in this study.Colonies that were asymmetric as a result of growing adjacent to a physical obstruction were not included as our focus was on the incidence of competition among octocorals. Examples of the different effects are shown in Fig. 1.

The data from the surveys were analyzed using hierarchical log-linear tests in SPSS (Version 22) to investigate the relationship of colony proximity with colony height (height classes of 5−19 cm, 20−29 cm,30−39 cm and ≥40 cm), octocoral genus and site. The proximity data were analyzed with respect to octocoral genus instead of species due to the low numbers of colonies in any one species. The 4-way analysis could not be undertaken due to a large number of expected values below 5, and the data were analyzed using three 3-way log-linear analyses. (The 4-wayanalysis yielded similar results, Table S1 in the Supplement at www.int-res.com/articles/suppl/m546p085_supp.pdf)

Colonies were divided into interaction types. Those in close proximity to a neighbor (=cases in which a colon's branches were within 5 cm of another octocoral colony (1) or had based within 5 cm of another colony (2) and those more distantly spaced (0).

Only the octocorals with an asymmetric colony form attributable to the presence of another octocoral were scored as asymmetric (1) in this study. All other colonies including those that were asymmetric as a result of growing adjacent to a physical obstruction were scored (0)